Absolute laserspektrometrische Stoffmengenanteilmessungen: Möglichkeiten für rückführbare Atemanalytik

Laser spectroscopic techniques such as tunable diode laser absorption spectroscopy (TDLAS), quantum cascade laser absorption spectroscopy (QCLAS) and cavity ring down spectroscopy (CDRS) have been shown to be capable of performing absolute amount fraction measurements of gas species such as CO2 and CO. These techniques have been proven to be very sensitive, selective and have real-time responses. The aim of this work was to: perform absolute amount fraction measurements of breath gas species using TDLAS, QCLAS and CRDS, reliably quantify breath gas species, address metrological data quality objectives, i.e. uncertainty and traceability issues, as well as define and reduce the uncertainty of amount fraction results from the typical 10 % to levels suitable to fit breath analysis purposes, 5 % and below. Thus, aiming at traceable amount fraction results, measurements have been performed using TDLAS, QCLAS and CRDS based on the absolute method TILSAM. GUM compliant uncertainty budgets for spectrometric amount fraction results were developed. TDLAS in combination with single-pass and multipass gas cells has been used to perform absolute measurements of the CO2 amount fractions. To check the TDL spectrometer for its feasibility for absolute amount fraction measurements and to be operated on the basis of the TILSAM method, gravimetric gas mixtures of CO2 in the range of 20 to 60 mmol•mol-1 were quantified. At the 50 mmol•mol-1 level (exhaled breath level) the relative standard uncertainties of the spectrometric CO2 amount fraction results are in the ±0.7 % range. The intra-pulse mode QCLAS has been utilized to measure absolute CO amount fractions at the 100 µmol•mol-1 and 1000 µmol•mol-1 levels based on the TILSAM method. Although, not at the exhaled breath level of 1-3 µmol•mol-1, the feasibility of intra-pulse mode QCLAS for CO measurements has been shown. The standard uncertainty of the CO amount fraction results, limited by the uncertainties of the line strengths used which were in the range of 2-5 % relative, are in the range of ±2.3 % relative. A CRDS spectrometer has been used to carry out absolute CO2 amount fraction measurements referring to the TILSAM method. The spectrometric results were in good agreement with the respective gravimetric reference values. The standard uncertainties of the CO2 amount fraction results, also limited by the uncertainty of the used line strength, were in the range of ±2.1 % relative. In a separate measurement, it has been shown in coperation with other partners that CO amount fractions in the nmol•mol-1 levels can be quantified using CRDS. It has been found that the TILSAM method suffers from the unavailability of traceable line data. Thus, line strengths and broadening coefficients of CO2 in the ro-vibrational band around 2 µm have been measured. The derived line data are in agreement to a high degree with published data. Compared to literature, improved GUM compliant standard uncertainties in the ±0.6 % range for the measured line strengths have been reported. The validity of the absolute method, TILSAM, has been further proven in a measurement campaign. The TDLAS-based quantifications were performed on CO2 at the 300 and 500 µmol•mol-1 level. The spectrometric results from the different laboratories were in good agreement, expressed by a degree of equivalence being in the 1 % range, with the respective comparison reference values (CRVs).Laserbasierte Spektroskopietechniken, wie z.B. die abstimmbare Diodenlaser-Absorptionsspektroskopie (TDLAS), die Quantenkaskadenlaser-Absorptionsspektroskopie (QCLAS) oder die "Cavity Ring-Down" Spektroskopie (CRDS) haben gezeigt, dass sie in der Lage sind, absolute Gaskonzentrationen von molekularen Spezies wie CO2 oder CO zu messen. Diese Laser-basierten Techniken sind sehr nachweisempfindlich, selektiv und können in „Real-Time-Response“ arbeiten. Das Ziel dieser Arbeit war es, absolute Stoffmengenanteile von Molekülspezies in Gasgemischen mit Hilfe von TDLAS, QCLAS und CRDS zu messen, zuverlässig zu quantifizieren und dabei messtechnische Datenqualitätsmerkmale, wie Messunsicherheit und Rückführbarkeit zu adressieren. Hintergrund für die Aufgabenstellung war es, die Anwendung dieser Spektroskopietechniken und der entwickelten Analyseverfahren in der Atemanalytik vorzubereiten. Messunsicherheiten sollten hierzu definiert und ggf. verringert werden. Die Unsicherheit der bestimmten Stoffmengenanteile konnte dabei von typischen 10% auf ein Niveau von 5 % und weniger reduziert werden, was für Atemanalysezwecke ausreichend ist. Die mittels TDLAS, QCLAS und CRDS ausgeführte Stoffmengenanteilsbestimmung basierte auf der sog. TILSAM-Methode. GUM-konforme Unsicherheitsbudgets für spektrometrische Stoffmengenanteilsmessungen wurden entwickelt. Um absolute Messungen von CO2 Stoffmengeanteile durchführen zu können, wurden Single-Pass- und Multi-Pass-Gaszellen in Kombination mit TDLAS verwendet. Zur Überprüfung des Einsatzes des TDLAS-Spektrometers für die Machbarkeit von absoluten Stoffmengenanteilsmessungen, die auf der Grundlage des TILSAM-Verfahrens durchgeführt werden, wurden gravimetrisch hergestellte Gasgemische von CO2 in Stickstoff im Bereich von 20 bis 60 mmol•mol-1 CO2 quantifiziert. Auf der 50 mmol•mol-1 Ebene (Atemluftkonzentration) konnte eine relative Standardmessunsicherheit der spektrometrischen CO2-Bestimmung von ± 0,7% demonstriert werden. Intrapuls-QCLAS wurde verwendet, um absolute CO-Konzentrationen im Bereich von 100 µmol•mol-1 bis 1000 µmol•mol-1 gemäß der TILSAM-Methode zu messen. Damit konnte die Machbarkeit der Intrapuls-QCLAS für absolute CO-Stoffmengenmessungen gezeigt werden. Die relative Standardmessunsicherheit der bestimmten CO-Stoffmengenanteile ist durch die Unsicherheiten der Eingangsgröße Linienstärke limitiert, die mit 2-5% spezifiziert waren, und lag damit im Bereich von ± 2.3%. Die Güte der spektrometrisch mit TDLAS und QCLAS bestimmten Stoffmengenanteile wurde anhand eines Vergleiches mit jeweiligen gravimetrischen Referenzwerten bestimmt. Darüber hinaus wurde auch ein CRDS-Spektrometer zur Durchführung absoluter CO2-Stoffmengenanteilsmessungen auf Basis der TILSAM-Methode eingesetzt. Die spektrometrisch erzielten Ergebnisse waren in guter Übereinstimmung mit den jeweiligen gravimetrischen Referenzwerten. Die relative Standardmessunsicherheit der CO2-Stoffmengenanteile wurde ebenfalls durch die Unsicherheit der verwendeten Linienstärke beschränkt, und lag im Bereich von ±2,1%. Da bekannt war, dass die Anwendung der TILSAM-Methode durch die Nichtverfügbarkeit von rückgeführten Spektralliniendaten, wie die Linienstärke, beschränkt ist, wurden Linienstärken und Verbreiterungskoeffizienten von CO2 auch im Rahmen dieser Arbeit bestimmt. Dafür wurden Absorptionslinien im ro-vibronischen Kombinationsschwingungsband von CO2 um 2 µm ausgewählt. Die so abgeleiteten Liniendaten stimmen zu einem hohen Grad mit den veröffentlichten Literaturdaten überein. Im Vergleich zu diesen werden die im Rahmen dieser Arbeit ermittelten Daten aber mit einem GUM-konformen Unsicherheitsbudget angegeben. Die entsprechenden Standardmessunsicherheiten der Linienstärken liegen dabei im Bereich von ± 0,6%. Die in dieser Arbeit weiterentwickelte TILSAM-Methode konnte darüber hinaus in einer internationalen Messkampagne eingesetzt werden. Die TDLAS-basierte Quantifizierung von CO2 wurde bei 300 und 500 µmol•mol-1 durchgeführt. Die spektrometrisch erzielten Ergebnisse aus den verschiedenen Labors waren in guter Übereinstimmung mit dem Referenzwert, ausgedrückt durch einen Grad der Übereinstimmung (Degree-of-Equivalence) im Bereich von 1%

Towards an Optical Gas Standard for Traceable Calibration-Free and Direct NO₂ Concentration Measurements

We report a direct tunable diode laser absorption spectroscopy (dTDLAS) instrument developed for NO₂ concentration measurements without chemical pre-conversion, operated as an Optical Gas Standard (OGS). An OGS is a dTDLAS instrument that can deliver gas species amount fractions (concentrations), without any previous or routine calibration, which are directly traceable to the international system of units (SI). Here, we report NO₂ amount fraction quantification in the range of 100–1000 µmol/mol to demonstrate the current capability of the instrument as an OGS for car exhaust gas application. Nitrogen dioxide amount fraction results delivered by the instrument are in good agreement with certified values of reference gas mixtures, validating the capability of the dTDLAS-OGS for calibration-free NO₂ measurements. As opposed to the standard reference method (SRM) based on chemiluminescence detection (CLD) where NO₂ is indirectly measured after conversion to NO, titration with O₃ and the detection of the resulting fluorescence, a dTDLAS-OGS instrument has the benefit of directly measuring NO₂ without distorting or delaying conversion processes. Therefore, it complements the SRM and can perform fast and traceable measurements, and side-by-side calibrations of other NO₂ gas analyzers operating in the field. The relative standard uncertainty of the NO₂ results reported in this paper is 5.1% (k = 1, which is dominated (98%) by the NO₂ line strength), the repeatability of the results at 982.6 µmol/mol is 0.1%, the response time of the instrument is 0.5 s, and the detection limit is 825 nmol/mol at a time resolution of 86 s

Towards a Fast, Open-Path Laser Hygrometer for Airborne Eddy Covariance Measurements

Water vapor fluxes play a key role in the energy budget of the atmosphere, and better flux measurements are needed to improve our understanding of the formation of clouds and storms. Large-scale measurements of these fluxes are possible by employing the eddy correlation (EC) method from an aircraft. A hygrometer used for such measurements needs to deliver a temporal resolution of at least 10 Hz while reliably operating in the harsh conditions on the exterior of an aircraft. Here, we present a design concept for a calibration-free, first-principles, open-path dTDLAS hygrometer with a planar, circular and rotationally symmetric multipass cell with new, angled coupling optics. From our measurements, the uncertainty of the instrument is estimated to be below 4.5% (coverage factor k = 1). A static intercomparison between a dTDLAS prototype of the new optics setup and a traceable dew point mirror hygrometer was conducted and showed a systematic relative deviation of 2.6% with a maximal relative error of 2.2%. Combined with a precision of around 1 ppm H₂O at tropospheric conditions, the newly designed setup fulfills the static precision and accuracy requirements of the proposed airborne EC hygrometer

H₂O Collisional Broadening Coefficients at 1.37 µm and Their Temperature Dependence: A Metrology Approach

We report self- and air collisional broadening coefficients for the H₂O line at 7299.43 cm⁻¹ and corresponding temperature coefficients for a temperature range spanning 293–573 K. New laser spectroscopic setups specifically designed for this purpose have been developed and are described. The line parameters reported here are in good agreement with those values reported in the HITRAN 2020 database, but the uncertainties have been reduced by factors of about 4, 1.3 and 4.4 for the self-broadening coefficient, air broadening coefficient and the temperature exponent of air broadening, respectively. Further, we combined our measurement approach with metrological data quality objectives, addressing the traceability of the results to the international system of units (SI) and evaluated the uncertainties following the guide to the expression of uncertainty in measurement (GUM)

Need for a protocol for performance evaluation of the gas analyzers used in biomethane conformity assessment

Biomethane may contain trace components that can have adverse effects on gas vehicles performances and on the pipelines when injected in the gas grid. Biomethane quality assurance against specifications is therefore crucial for the integrity of the end-users’ appliances. Analytical methods used to assess biomethane conformity assessment must be validated properly and possibly, new methods specifically for biomethane should be developed. This paper provides an overview of the biomethane quality assurance infrastructure and the challenges faced with focus on sampling, analysis methods, reference gas mixtures, and performance evaluation. Currently, requirements for analytical method validation and fit-for-purpose assessments do not exist for biomethane. The industry is in urgent need of a protocol to evaluate the fit-for-purpose of methods in a harmonized manner. Reference gas mixtures to check the accuracy of the instrument and to determine the traceability of the measurement are also urgently required. The project has received funding from the European Partnership on Metrology, co-financed by European Union Horizon Europe Research and Innovation Program and from the Participating States.Funder name: European Partnership on Metrology, Funder ID: 10.13039/100019599, Grant number: 21NRM04 BiometCAP.</p

Optical Path Length Calibration: A Standard Approach for Use in Absorption Cell-Based IR-Spectrometric Gas Analysis

We employed a comparison method to determine the optical path length of gas cells which can be used in spectroscopic setup based on laser absorption spectroscopy or FTIR. The method is based on absorption spectroscopy itself. A reference gas cell, whose length is a priori known and desirably traceable to the international system of units (SI), and a gas mixture are used to calibrate the path length of a cell under test. By comparing spectra derived from pressure-dependent measurements on the two cells, the path length of the gas cell under test is determined. The method relies neither on the knowledge of the gas concentration nor on the line strength parameter of the probed transition which is very rarely traceable to the SI and of which the uncertainty is often relatively large. The method is flexible such that any infrared light source and infrared active molecule with isolated lines can be used. We elaborate on the method, substantiate the method by reporting results of this calibration procedure applied to multipass and single pass gas cells of lengths from 0.38 m to 21 m, and compare this to other methods. The relative combined uncertainty of the path length results determined using the comparison method was found to be in the ±0.4% range